First off, let me say I have no experience in this area, but while reading this thread, I had a thought -- could you use a technique similar to the ones used to combat votebots? Since it seems you want your form to interface with a human and not a computer script/program, how about generating a confirmation image which must be reentered by the purchaser? I can't imagine the other site will want to hire people to sit around and reenter images as your product is purchased.